| Based on references involved the influences of very fast transient over-voltage (VFTO) caused by disconnector closing and switching off charging current of bus on gas insulated switch-gear (GIS) and the adjacent device, the key technique and technical path of investigation are found. The concerned research works of VFTO is investigated.A new transient circuit model to the question of the steep front and high amplitude value of VFTO is proposed based on general transient circuit model. The numeric simulation that disconnector switched off short bus has been proceeded by using above circuit model and the comparison of results with the one based on EMTP are given. The parameters of elements and equivalent circuit model in GIS are determined based on comparison analysis between test and numeric simulation, which has also provided a kind of valuable reference for simulating actual circuit. Three components of VFTO which are 2MHz, 10MHz and 40MHZ frequency components are obtained from Discrete Fourier Transformation, in which 40 MHz component plays a key role in GIS insulation.High frequency characteristic of epoxy dielectrics is studied based on experiment. In term of least square method (LSM), polynomial fitting curves of epoxy permittivity with respect to frequency under VFTO are obtained according to experimental data. By solving the maximum of above fitting curve, high frequency dielectric parameters, light frequency permittivity and relaxation time, are evaluated.According to lighting impulse voltage characteristic, the microscopic parameters of dielectric are introduced and differential equations including light frequency permittivity and relaxation time are firstly deduced. Duhamel integration method is introduced to express the response caused by lighting impulse voltage. Potential distribution curves at representative time and electric field intensity curves along with disc spacer surface are shown. The calculation results have shown that not only tangential field intensity but also stronger normal field intensity are found on surface of spacer. The normal field intensity effects on dielectric may bring the slip discharge along the surface of disc spacer.The mathematic models of frequency domain with complex permittivity ' are deduced to calculate transient electric field by combining macroscopic with microcosmic parameters. Potential distribution, electric intensity distribution, Tangential intensity e, and normal fieldintensity e,, curves along the radial direction on interface of spacer are illustrated under VFTO. Insulation performance of gas SF6 and disc spacer is investigated.Multi-conductor transmission line (MTL) model of transformer winding is deduced under VFTO, in which the mathematical equations and boundary conditions for calculating over-voltage in windings are given. The over-voltages in winding are solved by using finite different time domain (FDTD) method. Comparison between MTL and traditional method are given. The calculation results have shown that the tendency of over-voltage distribution curves by using MTL model is accordant with by traditional improper eigenvalue method. However, partial oscillation has been observed in windings by FDTD, which can't be found by improper eigenvalue method of lump parameters. In addition, improper eigenvalue method can only solve the over-voltage distribution of terminals. It has been shown that FDTD method, in which a turn is regarded as calculated unit, is a virtual method for calculating over-voltage distribution of transformer windings.
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